This year's flu epidemic has already overwhelmed hospital emergency rooms and claimed 20 children's lives. It could be better contained, not just by hand-washing and flu shots, but by tackling a glaring public policy gap: lack of sick leave.

We often hear about the workplace no-show problem known as absenteeism. But an even bigger problem in a flu-infested season like this one is what some are calling "presenteeism." That's when people show up for work when they shouldn't, because they're sick.

Those who do it are not necessarily compulsive about their jobs. They have no doubt heard the same warnings the rest of us have, to stay home. But many simply can't afford to. Nearly one-third of U.S. workers, or close to 42 million people, according to the Bureau of Labor Statistics, have no paid sick time off. Among part-time workers, only one in four has any.

This is an important but overlooked aspect of why influenza can so quickly reach epidemic proportions.

While absenteeism accounts for 22 percent of lost workplace productivity, "presenteeism" accounts for 78 percent, because sick workers infect others, according to the Center for Worklife Law at the University of California's Hastings Law School. That, it says, costs the U.S. economy $180 billion a year. In 2009, 5 million cases of the H1N1 flu resulted from a lack of paid sick days, according to a study in the American Journal of Public Health.

You would think that with such consequences, we would have tackled the problem at a national level. Yet the United States is one of the only developed nations that does not guarantee workers some paid sick leave.

By any measure, sick workers on the job are a problem. The National Partnership for Women and Families found that sick employees who come to work are half as productive as usual. It also estimates the medical costs for sick workers infected by their co-workers at $102.4 million a year.

Low-wage workers have it worst, and often with a ripple effect. An estimated 80 percent of them get no sick leave, and some are clustered in industries such as food service, where the spread of illness is high. According to the Partnership, 79 percent of food workers say they have no paid sick time.

A bill to require paid sick time off for employers of 15 or more people has repeatedly been introduced but failed to pass in Congress. Called the Healthy Families Act, it would require such employers to offer at least one hour of paid sick time for every 30 hours worked -- for a maximum mandate of 56 hours in a year.

Last year, Iowa's Democratic senator, Tom Harkin, proposed the Rebuild America Act, with a provision guaranteeing all workers the right to earn up to seven sick days in a year. That, too, failed to get momentum.

The Institute for Women's Policy Research, which explores issues related to women and families, estimates that reducing just the spread of the flu in workplaces could save $738.5 million a year. Even if only to protect their bottom line, employers should be providing sick leave as a matter of course.

It would be one thing if there were credible fears of employees abusing their sick leave. But the Bureau of Labor Statistics finds that most full-time private-sector workers don't even use all the sick leave they earned.

Some cities and one state are no longer waiting for the federal government to act. They're passing their own sick leave requirements. San Francisco, Seattle and the District of Columbia all have laws guaranteeing paid sick days, as does Connecticut.

That's fine as a stopgap measure. But a health crisis such as this one makes painfully clear that it is time for a federal standard. This is not just an employment issue but a public health one.

When more than 40 million private-sector workers have to weigh the cost of staying home when they get sick, because it could mean sacrificing meals, rent money or other essentials, they unwittingly put other employees at risk. This is one of those cases where companies clearly have to be pushed to do right, not just by their workers but by their own bottom lines.